"Right before Christmas, my little Ace suffered a herniated disk," Underwood wrote on Wednesday. “He was completely paralyzed in his back end for a couple of days and, with rest, soon got mobility back in his right side...his left side is slowly getting better but we have a long way to go."

"He's taking his physical therapy like a champ!" continued the GRAMMY winner, before adding, "And, of course, this is too adorable not to post!"